1. Field of the Invention
This invention pertains to the removal of sulfur values in the treatment of alkali metal sulfide and carbonate mixtures. It particularly relates to a method wherein the alkali metal sulfide is converted to the corresponding alkali metal carbonate and the sulfur is recoverable as a salable product. In a particularly preferred aspect, the invention relates to the treatment of a melt comprising a mixture of alkali metal sulfide and carbonate, the melt being obtained from a process wherein a carbonaceous material, such as coal, is reacted in a molten alkali metal carbonate bath.
2. Prior Art
There are several sources of mixed alkali metal salts such as, for example, mixtures of alkali metal sulfides and carbonates, which advantageously are treated to recover the sulfur content and produce a substantially pure alkali metal carbonate. One such source is the pulp and paper industry, wherein a cellulose is manufactured by digestion of wood with alkali metal sulfides or sulfite. In such a process a bleed stream digestion liquor is drawn off, concentrated and burned producing a molten salt mixture which generally consists substantially of sodium carbonate and sodium sulfide. Various methods have been suggested for treating such molten salts.
More recently, it has been suggested that carbonaceous materials such as coal, coal tar, oil shale, petroleum coke, and petroleum residuums, be decomposed in a molten alkali metal carbonate bath to recover the resultant valuable gaseous products. The feed materials generally contain sulfur compounds, which react with and are retained in the molten bath. In addition, such materials, particularly in the case of coal, also contain significant amounts of inorganic ash constituents, which also are retained in the molten bath. Therefore, it is necessary to periodically (or continuously) remove a portion of the molten alkali metal carbonate bath and treat it to remove the retained ash constituents and sulfur, and permit the return of alkali metal carbonate to the bath for further use.
In U.S. Pat. No. 2,094,070, Hultman et al., there is disclosed a process for recovering H.sub.2 S from gases. Broadly, the process comprises contacting an H.sub.2 S-containing gas stream with a solution of alkali metal carbonate, which absorbs the hydrogen sulfide. The solution then is treated with a sufficient amount of carbon dioxide to convert the carbonate into bicarbonate without removal of the hydrogen sulfide. The treated solution is boiled under a vacuum to expel the hydrogen sulfide and steam. The boiling is then continued at a higher temperature and pressure to convert the bicarbonate back to the carbonate for recycle to absorb more H.sub.2 S.
U.S. Pat. No. 2,496,550, Larsson et al., discloses a process for the recovery of alkali metal salts from the waste liquors from the production of cellulose by the digestion of the wood with an alkali metal salt such as alkali metal sulfites and bisulfates. The process comprises evaporating and burning the waste liquor to form a molten mass containing alkali and sulfur compounds followed by dissolving the molten mass in a solvent. Thereafter carbon dioxide is introduced into the solution to form alkali metal bicarbonate in an amount sufficient to approach the limit of solubility of the bicarbonate in the solution. The solution is then heated to eliminate the hydrogen sulfide formed therein. After removal of the hydrogen sulfide, the solution is treated with additional carbon dioxide to crystallize alkali bicarbonate, which is recovered from the solution. Larsson et al further suggest that the solution, after removal of the alkali metal bicarbonate, be used as the solvent for dissolving the molten mass formed from burning the waste liquor.
U.S. Pat. No. 2,675,297, Gray et al., relates to the treatment of aqueous solutions of sodium sulfide for conversion of the sulfides to salts of carbonic acid and the liberation of hydrogen sulfide. Patentees suggest subjecting a solution containing sodium sulfide to a plurality of carbonaceous treatments with gaseous carbon dioxide at an elevated temperature and pressure, each carbonation treatment being followed by steam stripping under a vacuum to remove volatile hydrogen sulfide in a concentrated form, thereby producing a solution having an enhanced content of sodium salts of carbonic acid substantially free from sulfide.
In U.S. Pat. No. 3,567,377, Lefrancois et al, there is disclosed a process for the recovery of sulfur values from sulfur-bearing materials. In accordance with the process disclosed therein, a sulfur-containing carbonaceous material is contacted in the presence of a reducing gas with a molten medium comprising an alkali metal carbonate to convert the sulfur to an alkali metal sulfide. The molten medium containing the absorbed alkali metal sulfide is mixed with an aqueous solution of the acid salt of the alkali metal carbonate. The resulting solution is filtered to remove any solids contained therein and then reacted with carbon dioxide to form hydrogen sulfide as a gaseous product of the reaction.
U.S. Pat. No. 3,508,863, Kiminki et al, discloses a process for the preparation of sodium carbonate monohydrate from a soda smelt solution. The process is directed toward the treatment of a smelt solution arrived at by burning spent pulping liquor. The smelt solution is precarbonated with gases containing carbon dioxide to form dissolved sodium bicarbonate followed by evaporation of the precarbonated solution together with added sodium bicarbonate to remove all the sulfides as hydrogen sulfide together with water vapor and to form crystalline sodium carbonate monohydrate for recovery.
Other patents relating to the treatment of alkali metal sulfides are U.S. Pat. Nos. 1,945,163 (Rosenstein et al.); 2,730,445 (Sivola); 3,438,728 (Grantham); 3,574,543 (Heredy); and 3,867,514 (Moore).
While each of the foregoing processes offer certain advantages, none has proven entirely satisfactory. Some are disadvantageous in that a multitude of steps are required. In others no provision is made for obtaining the necessary carbon dioxide from the process itself, thus raising the operating cost, while others create difficulties owing to corrosion and very high amounts of gases to be handled. Several of the processes require use of excessive amounts of steam, and others require use of heat exchangers which are subject to plugging and scaling due to the deposition of solid materials from solution. A further disadvantage common to several of the foregoing processes is that the recovered alkali metal carbonate is not obtained in a substantially pure form and contains sulfates and sulfides. In still others, noxious sulfur-containing gases are evolved to the atmosphere. Obviously, therefore, there is still a need for an improved economical process for the treatment of mixtures of alkali metal sulfides and carbonates for the removal of sulfur values therefrom.